How to make a mask for voronoi texture (with randomness)?
The new Voronoi texture conveniently gives you a 'Position' output, which gives you (in 2D ) the X and Y coordinate of the center of a shading-point's cell.
So, if you don't want to cut your circle-'pixels' in half, you can use the distance of the whole cell from the texture space's origin to create a mask of whole cells, rather than the texture space itself.

I've scaled the whole texture down, so the cell-resolution is high enough to capture some circular curvature.